Discharge/Charge Curves of Lithium - Sulfur Batteries

[Jake S.]

Hi,

 

What does this dischrage/charge curve represent and what is the chemistry behind it?

What do the curves represent and what does gravimetric and areal capacity mean?

 

Thank you very much.

 

 

 

 

[studiot]

1 hour ago, Jake S. said:

Hi,

What does this dischrage/charge curve represent and what is the chemistry behind it?

What do the curves represent and what does gravimetric and areal capacity mean?

Thank you very much.

 

 

I assume you are studying this because you are interested in using lithium - sulphur rechargeable batteries as opposed to the chemistry of them  ?

The graphs are meant for users / manufacturers of these batteries.

 

Remembering that a battery is a voltage source, which means that it theoretically provides a fixed voltage regardless of the current drawn.

Real world batteries, including the Li - S ones suffer lowering of the voltage as the current drawn increases until the curve 'turn sharply downwards' which represents the  limit of their usefulness as they can supply no greater current.

In the published graphs the current is given as a current density in terms of cross sectional area of the 'plates' (the aereal scale) or the sulphur density (gravimetric) which has a practical chemical limit.
If you assume unit cross section then a battery of that cross section would supply the scale current at the scale voltage.

This enables manufacturers to design the size of the battery and users to estimate the discharge time.

 

When charging, of course, the charging supply voltage has to exceed that of the battery terminals.

So the charging curves show how much voltage is required to charge the cell and also at a given charging voltage, the charging current which when multiplied by the charging time gives the charge stored. The charge stored is called the capacity.

 

Does this help  ?

 

 

[Jake S.]

@studiotThanks.  Do you know what experiments scientists run through to get the discharge/ charge graphs. 

[studiot]

7 hours ago, Jake S. said:

@studiotThanks.  Do you know what experiments scientists run through to get the discharge/ charge graphs. 

Obviously the experimental scientists will know how much sulphur  and other compounds they put into battery.
They will also know its dimensions.

Current density etc are not directly measurable, currents and voltages are.

So they will use measure the current and voltage using a suitable load to draw the currents over the range involved.

(often this is a specialist piece of electronic kit known as a contant currrent load  -  these can be quite expensive)

Then this can be combined with the known data to produce the graphs.

 

https://www.google.co.uk/search?source=hp&ei=YCtKX7rSCt2GjLsP9tSc8As&q=constant+current+load&oq=constant+current+load&gs_lcp=CgZwc3ktYWIQAzICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgYIABAWEB4yBggAEBYQHjoOCC4QsQMQxwEQowIQkwI6BQgAELEDOg4ILhCxAxCDARDHARCjAjoLCC4QsQMQxwEQowI6CAgAELEDEIMBOggILhCxAxCDAToFCC4QsQM6CwguEMcBEKMCEJMCOggILhDHARCjAjoICC4QsQMQkwJQpgNY_hxgziBoAHAAeACAAcoDiAGYMZIBCTAuMS42LjYuNpgBAKABAaoBB2d3cy13aXo&sclient=psy-ab&ved=0ahUKEwi6gvHil8DrAhVdA2MBHXYqB74Q4dUDCAg&uact=5

 

Of course, one would need to measure a sufficient number of batteries to obtain a statistical average.

These could then be reliably compared for different battery geometries (shapes and sizes).

A battery needs to do two thing internally.

When charging you need to distribute the charge evenly, without creating localised hotspots -  either chemical or thermal.

Equally when discharging you need to collect the charge evenly without damaging the battery.

 

Edited August 29, 2020 by studiot